The present invention, for the purpose of simplicity, will be discussed herein particularly in relation to installation of subsea silos intended to enable positioning of subsea well heads at a desired level below the seabed surface or mud line to thereby protect the well head from damage. It is not intended however to limit the present invention solely to subsea silos for well head installation, it being obvious that the present invention is functional in any environment where a protective subsea enclosure may be desired for apparatus of any suitable character. The spirit and scope of the present invention therefore extends to installation of particular enclosures other then subsea well head silos and to methods for installation of the same within the spirit and scope hereof.
It has now become a wide spread practice to drill oil producing wells in shallow offshore sea areas. In sea areas where ice bergs are present, danger to subsea equipment is obvious. Aside from the possibilty of showing the seabed during ice berg movement, they also tumble from time to time as the surface portion melts and the center of gravity changes. During such tumbling ice portions can contact the seabed, developing deep scouring. In the Beaufort Sea for example the water is shallow and there is a serious hazard in the form of floating ice which tends to accumulate. This floating ice may develop into ice ridges which not only accumulate above the water but also develop a substantial submerged section referred to as an ice keel.
The ice ridges and ice keels tend to drift responsive to wind and current and as they are driven relative to shallow areas, they may scour the sea floor. Thus, it has become necessary for all companys operating in the Beaufort Sea where sheet ice is present to provide means for protecting the subsea well head equipment including blowout preventors (BOP), well heads, etc. from the risk of ice damage by the scouring effect of moving ice ridges and ice keels. It has been found desirable therefore to locate subsea well heads and BOP stacks beneath the point of the seafloor of known ice ridge scour. In the past the required depth of well head location was achieved by dredging a large area of the seabed to a depth below known iceberg or ice keel scouring (known as a "glory hole") and setting the well head and BOP stack in this depression on the seabed.
The above method is extremely costly and requires the dredging of large quantities of material with a seagoing dredger of high capacity, or operating the dredge head airlift of a dredging ship. A large "clam shell" dredge may also be employed to dig glory holes, but represent considerable expense. An example of a prior system is described in Canadian Pat. No. 995,583 issued Aug. 24, 1976. That system includes a caisson embedded in the seafloor by methods such as driving, jetting or a combination of the two. The upper region of this caisson includes a plurality of horizontally connected circular segments joined by breakaway joints. In this manner, when an upper portion of the caisson is contacted by an icemass, the entire casing is not damaged or deformed, but only a particular segment may be broken away. With regard to generally related methods and apparatus U.S. Pat. Nos. 4,318,641 and 4,432,671 teach hydrostatic sinking of anchors in waterbottoms.